Fix models that have mix of draco and non-draco attributes

[lilleyse]

Before	After

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ggetz]

@sanjeetsuhag Can you please review?

[sanjeetsuhag]

The code changes look good. @lilleyse Is this a regression from the Model.js implementation? If so, should we call out a fix in CHANGES.md?

[lilleyse]

@sanjeetsuhag I'm not sure if the old Model.js had this problem, but I updated CHANGES.md anyways

[sanjeetsuhag]

Thanks @lilleyse!